The image below shows a clear view of minion positioning so that you may judge the distance for yourself. there are two buff beams in this image, one is the max length of a 4DU Buff Beam and the other is the max length of a 5 DU Buff Beam. there are 7 archers leading away from the mage each was summoned as close to each other as the game would let without moving them after summoning.

This image shows the healing in action. It turns out the mage minion heal radious is slightly less than the max length of a 5DU Buff Beam. Notice that archer 6 has heal numbers over him but archer 7 does not. No amount of gear or stats change the mages heal radious